mu: add fields/flags commands

Useful information for devising queries.

Directly generated from the source. Add manpages, too.

man/mu-flags.1

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+.TH MU FLAGS 1 "April 2022" "User Manuals"
+
+.SH NAME
+
+mu flags\-  list all message flags
+
+.SH SYNOPSIS
+
+.B mu flags [options]
+
+.SH DESCRIPTION
+
+\fBmu flags\fR is the \fBmu\fR command for showing a table of message flags and their properties, which is useful for devising queries.
+
+.SH OPTIONS
+
+Inherits common options from
+.BR mu(1)
+
+.SH BUGS
+
+Please report bugs if you find them:
+.BR https://github.com/djcb/mu/issues
+
+.SH AUTHOR
+
+Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+
+.SH "SEE ALSO"
+
+.BR mu (1)